This invention relates to speech communication by a cellular telephone operable in both a digital mode and in an analog mode of communication and, more particularly, to the further processing of a voice signal received during the analog mode of voice communication to provide to the received voice signal a digitally processed voice quality. This ensures a common perceived voice quality in both analog and digital modes of operation.
Cellular telephones may be constructed with a dual-mode operational capability wherein, in one mode, voice signals are communicated via an analog signal protocol and, in the second mode, the voice signals are communicated via digital signal compression. Some base stations operate in an analog mode while other base stations are able to operate in the digital mode. The dual-mode telephone is able to take advantage of the digital signal protocol when offered by a base station, while falling back to the analog signal protocol for base stations offering only the analog mode of communication. For vehicles traveling from the region of one base station to the region of a second base station, there is a hand-off procedure wherein the cellular telephone automatically tunes to a new assigned communications channel and, in addition, may undergo a switching between the digital and the analog modes of communication.
From the point of view of a person operating the cellular telephone, such a hand-off procedure including switching between digital and analog modes is accomplished automatically and should, therefore, be of no concern to the person operating the cellular telephone. However, there is a distinct identifiable quality to digitally processed voice signals which is different from a distinct identifiable quality to analog processed voice signals. The perceptual differences are much more pronounced when background noise, such as car noise or babble noise, is present. As a result, a person operating the cellular telephone becomes aware of a hand-off in which the telephone switches between digital and analog modes of communication.
A problem arises in that many telephone customers react adversely to the change in perceived quality of the voice communication during a switching between the digital and the analog modes of communication. It has been observed that some telephone customers actually return their dual-mode telephones to the telephone supplier because the customers believe that such change in voice quality is an indication of faulty telephone operation. It appears that people, in the use of telephones, expect a uniform quality and type of sound to which they may have grown accustomed.